Fender BASSMAN 1200 Service Manual page 5

BASSMAN 1200
(This is the model name for warranty claims)
Power Amplifier
The power amplifier employs class AB topology in the pre-driver circuit Q8-Q11. Quiescent current is established with
transistors Q12-Q13. Driver transistors Q16-Q17 provide drive for Q23-Q26, and Q31-Q34. Transistors Q19,Q21, and
Q27-Q30 provide positive "on demand" supply-rail boost. Transistors Q20, Q22, and Q35-Q38 provide the same
function for the negative rail.
Transistor Q18 provides "charge depletion" of Q23-Q26 and Q31-Q34. This allows for the suppression of common
mode current that can occur at high frequencies.
Relay K1 and associated components reduce output-stage current gain to unity during power-up and over-temperature
events as monitored by thermistor RT1 on the main heatsink.
Power supply
Primary circuit inrush limiting is accomplished by TH1, a negative temperature coefficient thermistor.
The power supply employs a toroidal power transformer with only one tapped-main secondary. Two large bridge
rectifiers are used to obtain +/-47VDC and +/- 94VDC.
High voltage (356VDC) for vacuum tube sections is generated by half-wave multiplier comprised of CR92-CR95,
C125-C128.
Low voltage (+/-22VDC) and (+10~ +28VDC variable fan) is supplied by the synchronous rectifier system located on
the power supply break-away board. This synchronous system observes the input sine and allows Q81, Q85, and Q88
to be forced on until the input sine wave reaches the correct level as observed by Q83, Q87, and Q90. References
Q84 (variably controlled by RT2) CR811 and CR818 are employed. Regulators U21 and U22 reduce the +/-22V lines
to +/-16VDC
Heater voltage (24VDC for 2 series-connected 12AX7s in the preamp) is derived from the 32VDC supply (+/-
16VDC).. Excess voltage is dropped across across paralleled ballast resistors, R354-R356, on the preamp PCB.